Weird behavior ...

Hello and good evening. I just started receiving the following error when attempting to leave a comment via WordPress, however, if I attempt to comment via FB or Twitter, I do not see the error.

I would really appreciate your help in determining the cause of the following error:
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'wdcp_mcls_mailchimp_signup' not found or invalid function name in /nfs/c10/h03/mnt/150801/domains/ on line 403

Warning: Cannot modify header information - headers already sent by (output started at /nfs/c10/h03/mnt/150801/domains/ in /nfs/c10/h03/mnt/150801/domains/ on line 881

Note: If I press the 'Back' button in my browser, the comment will post and the error will be gone.

Thank you in advance for your assistance in this matter.

Matt Vaden